Metal oxide semiconductor (MOS) devices are one of the most popular devices in the modern semiconductor industry. An MOS field-effect transistor (MOSFET) generally has three contacts: a gate electrode, a source region and a drain region. The gate controls the current flowing through the transistor. More specifically, in a MOS transistor, current of a particular polarity (positive current for a p-channel MOSFET and negative current for an n-channel MOSFET) is driven from the source region to the drain region, through a channel under the control of the gate. FIG. 1 shows the cross section of an exemplary metal-gate MOS transistor having a metal gate 10 on top of an oxide 11, which is on top of a substrate 12 (thereby forming the "MOS" structure). The MOS transistor also includes a source 14 and a drain 16 formed in the substrate 12 having an opposite conductivity to that of the substrate. The source 14 and drain 16 are located at the opposing ends of the gate 10. A channel region 18 separates the source 14 and the drain 16, and is aligned substantially beneath the gate 10.
In operation, when a voltage is applied to the metal gate 10, the electric field formed causes charge in the channel region 18 to redistribute. For example, a positive voltage will tend to attract negative charge to the channel region 18. If the channel region 18 is normally p-type, then the attracted negative charge can invert the conductivity of the channel region to the n-type. The surface of the semiconductor substrate 12 between the source 14 and the drain 16 is thus inverted and forms a conductive channel thereon.
For the metal-gate structure shown in FIG. 1, the metal gate 10 is commonly made of a metal such as aluminum. In a typical process, due to the low melting point of aluminum, the aluminum must be deposited after the source region 14 and the drain region 16 are treated by a high temperature drive-in process. Further, the patterning of the metal gate 10 requires a positioning tolerance and adversely affects the packing density of the integrated circuit.
A polysilicon-gate structure was developed for overcoming the drawbacks of the metal gate structure. FIG. 2 shows the cross section of the polysilicon-gate structure. Owing to the high melting temperature of the polysilicon, the polysilicon gate 20 can be deposited prior to forming the source region 22 and the drain region 24. Furthermore, the doped polysilicon 20 has a low work function (often referred to as threshold voltage) compared to aluminum, thereby requiring less power and allowing the transistor to operate more quickly.
The disadvantage of the polysilicon-gate structure is that polysilicon generally has a resistivity higher than aluminum and forms a poor contact with aluminum interconnect. Therefore the "RC" time delay in charging the gate is much greater for polysilicon gates. Consequently, the formation of metal silicide layers on top of the polysilicon layers was developed, resulting in a gate structure referred to as a polycide gate shown in FIG. 3. The polycide has a much lower resistivity and forms a better contact with aluminum than polysilicon. To form the polycide, a polysilicon plate 32 is formed on a thin gate oxide 30. Then silicide 34 is formed by reacting a metal with the upper portion of the polysilicon plate 32.
A cap silicon nitride layer 36 is then deposited by low pressure chemical vapor deposition at a temperature of about 780.degree. C. to passivate the underlying silicide 34. Typically, prior to the formation of the silicon nitride layer 36, a rapid thermal oxidation step is added. This process may cause gaps or extrusions to form in the silicide 34 due to phase transition of the silicide at high temperature. The extrusions may result in short circuits.
What is needed is a new method of forming a polycide gate structure.